First, a little back story. About 3 weeks ago my fuel pump died and was replaced. I had also been having some issues with my transmission for an extended period of time. A week after the fuel pump replacement, I finally bit the bullet and got it in a transmission shop for repair.

I have a code reader which was, prior to the transmission overhaul, returning a code that read “transmission component slipping”. I got that code several times over a period of about a month so I knew it was time to get it in for repair. Repair was completed week before last. Tuesday of last week I took a road trip to Kentucky. All seemed OK for most of the trip but coming home I needed to jump into the opening of a road train in order to get around a slower moving 18 wheeler. I floored the accelerator expecting passing gear but it never downshifted. Engine was speeding up, up to a point, but then it just would not go over about 78mph. The “Service Engine Soon” light also came back on concurrent with this.

Considering the previous code regarding the transmission, when I got home, I plugged in the code reader fully expecting to see a transmission related code. To my surprise, the code was “P0302 Cylinder 2 Misfire Detected”.

Then, a day later on a rural road traveling about 40-45mph I tried again to get it to go into passing gear with the accelerator on the floor. NO joy! It would not downshift! The engine was revving up pretty high but it would not downshift. At the same time the “Service Engine Soon” light came back on. Read the codes again and this time they were different, again, nothing to do with the transmission. This time the codes were “P0174 Fuel Mixture Imbalance” and “P0171 Fuel Trim System Lean Bank 1”.

I don’t know what to think about this. I was convinced the transmission was slipping, even worse than it did before the overhaul. With the codes being only engine related, now I just don’t know. I’m hoping one of our knowledgeable members can unravel this mystery. Could these engine related codes be preventing the transmission from downshifting?

I have a slightly younger one for my business. I it up to about 230,000 miles and it has been amazingly reliable. As far as your situation here are a few things I would suggest.

1- Run Chevron Techron in several tanks in a row using full dosage or 50% more.

2- Get some MAF sensor cleaner and clean the MAF sensor. It sits behind the air filter and measures how much air is entering the engine. You just spray the wire for about 10-15 seconds to clean it.

3-It may be O2 sensor related and you might start to get a code for that. If that happens, replace the sensor but make sure you soak it in PB Blaster or some other penetrating oil overnight several nights in a row. Those things are notorious for seizing in place.

4- If you haven't changes plugs and wires in a while replace them.

And yes, a poorly running engine can prevent the trans from shifting especially if it is not generating vacuum properly. I believe you have a 4L80E which is electronically controlled so the engine computer may be preventing the trans from shifting.

None of that means you DON'T have a transmission problem necessarily.

I didn’t think those vans were noted for longevity. Give consideration to the good $$ after bad equation. 20 years, likely close to 200k on the miles.

Not to be a downer, possible engine issues along with the tranny. Depending on use, maybe it’s just fine, shorter trips.

Cruiser,

Thanks for that insight and I will try the suggested things. One thing I will say though is that the engine isn't running poorly. Lacking those codes you would never know anything was amiss as it runs quietly and smoothly.

V6 or V8?

All those codes can be from lack of fuel, although p0302 by itself would make me think coil pack/spark first.
Misfires will feel like transmission slipping & can cause odd shifting depending on what the computer is thinking.
My first guess would be that they didn't get something plugged back in fully when they did the fuel pump or transmission. A good transmission shop would diagnose it at least.
Could be O2 sensor related
Could be some crud in the fuel rail from when they changed the pump.
This is where a good scan tool & a tech that knows how to use it saves a lot of diagnosis time & 'throw parts at it' $$.

Reason I'm asking about the engine is that if it has a 4.3 V6 those years had an issue with the distributor.

4.3's used a conventional distributor that is camshaft driven, not the electronic system that uses sensors in the 4.8/5.3/6.0 V8 engines for camshaft and crankshaft signals via those sensors.

I had a Sierra that developed a surge at cruise speeds and Service Engine Soon light on. All codes and indications were ignition related. Either replaced primary ignition items or swapped with a known good component, we were grabbing at straws. One of the techs at the dealership I last worked at remembered a TSB regarding excessive wear between the distributor driven gear and the camshaft drive gear. Bingo, pulled the distributor, saw the gear teeth were knife edged, pulled the camshaft and saw the same thing. All at less than 100k, I had a GMPP policy on that truck that covered the repairs.

But in the end it made sense, the failure and the codes. Fuel injection signal came from the ECM via pulses sent by the distributor, distributor signal was inaccurate.

It is a 350 V8 with 4 speed transmission.

Has the intake manifold gaskets ever been replaced on your chevy? If not I would be looking around the intake for vacuum/air leaks and or coolant leaks. Especially around Cylinder #2. GM had a big problems with their OEM intake gaskets around that time frame on the 350. Many times they end up with coolant in the oil. A UOA would show that even if its just a small coolant leak.

Check the EGR valve, possibly stuck open?

Check the MAF (mass air flow) sensor for being dirty, as well as the duct between the sensor and the engine for being part way off, torn, cracked, etc., and the air filter itself. This sensor has a big effect on when, how hard and sometimes whether the transmission will shift. It can also cause those lean codes.

I know it has a new fuel pump, but n-e-w doesn't always spell good. Check the fuel pressure (65 psi, absolute minimum 55) and volume.

I am assuming this intermittently happens correct? Most of the time it drives normally?

It sounds air/fuel/spark related which is why I suggest looking at those things. It's gonna be a process of elimination.

quote:

Originally posted by egregore:
Check the MAF (mass air flow) sensor for being dirty, as well as the duct between the sensor and the engine for being part way off, torn, cracked, etc., and the air filter itself. This sensor has a big effect on when, how hard and sometimes whether the transmission will shift. It can also cause those lean codes.

I know it has a new fuel pump, but n-e-w doesn't always spell good. Check the fuel pressure (65 psi, absolute minimum 55) and volume.

I concur, this is what I would do first.

Finally took the van to a highly recommended repair shop. They did a very thorough job diagnosing the issue. It turned out to be a failing "Spider" in the throttle body fuel injection system. This contraption actually looks like an electronic spider. There is a central control unit with 8 fairly long "legs" with injection nozzles on the end. In the last week or so before going in to the repair shop, the van was getting nearly impossible to start. The van had always been very quick to start but had started having to crank a lot and trying to fire and would eventually catch and start, thankfully before the battery wore down. Last week, it would not start on its own but needed a shot of starting fluid to get it to go.

This is the fuel injector assembly for that engine. It looks like the "face-sitter" from the Alien movie.



So this finally fixed it?